This invention concerns a process for isomerizing sulfur contaminated isomerizable hydrocarbons including paraffins and cycloparaffins. More particularly, this invention concerns a process for isomerizing an admixture of process streams, hereinafter referred to as the combined feed stream containing C.sub.4 -C.sub.7 hydrocarbons, hydrogen and sulfur utilizing a catalytic composite comprising a hydrogenation component selected from the Group VIII noble metals, a hydrogen form crystalline aluminosilicate, and a refractory inorganic oxide.
The isomerization of low molecular weight normal paraffins is well established in the art. This reaction is of considerable importance in the petroleum industry because of the substantially higher octane number of isoparaffins compared to their normal paraffin counterparts. Since gasoline blends require a distribution of boiling range materials, the isoparaffins in the C.sub.4 -C.sub.7 range are valuable blending components. A blend of various isomeric paraffins provides a gasoline which has a higher octane number than a gasoline consisting of normal paraffins. Isomerization is generally performed by passing light straight run (LSR) naphtha together with hydrogen through a reaction zone containing an isomerization catalyst. These LSR naphthas, containing primarily saturated C.sub.5 and C.sub.6 hydrocarbons, are commonly obtained from a refinery atmospheric distillation column and can contain sulfur compounds of various types in levels as high as 200 wt. ppm. As a result, conventional paraffin isomerization technology requries that feedstocks containing these levels of sulfur compounds be hydrotreated or otherwise desulfurized prior to exposure to the isomerization catalyst. However, this pretreatment of the feed represents additional capital and operating costs that can make upgrading LSR naphtha an uneconomical option. Thus, an isomerization process which can process sulfur containing feeds would result in a more efficiently produced and economically desirable high octane value product.
It has been the practice up until this time to isomerize paraffins to equilibrium mixtures of their branched chain isomers with a variety of catalysts. Friedel-Crafts catalysts, such as aluminum chloride, are known to be effective isomerization catalysts. Noble metals, such as platinum supported on halogenated alumina or silica alumina have also been used effectively to isomerize hydrocarbons. More recently, crystalline aluminosilicate zeolites which have shown catalytic activity have been effectively used in the isomerization of hydrocarbons. Both natural and synthetic crystalline alumino-silicates have been employed. Included among these are the Type X and Type Y zeolites as well as synthetic mordenite.
Specifically, the zeolites known as mordenites have received great attention. Mordenites are crystalline natural or synthetic zeolites of the alumino-silicate type; generally, they have a composition expressed in moles of oxide of EQU 1.0.+-.0.2 Na.sub.2 O.multidot.Al.sub.2 O.sub.3 .multidot.10.+-.0.5 SiO.sub.2 ;
the quantity of SiO.sub.2 may also be larger. Other alkali metals and/or alkaline earth metals may be substituted for all or part of the sodium.
In general, it has been found that the sodium form of mordenite is not particularly effective for isomerization of hydrocarbons and that replacing all or, for the greater part, the sodium cations with hydrogen ions yields the more advantageous hydrogen form mordenite. Conversion of the sodium form to the hydrogen form can be accomplished by a number of means. One method is the direct replacement of sodium ions with hydrogen ions using an acidified aqueous solution where the process of ion exchange is employed. Another method involves substitution of the sodium ions with ammonium ions followed by decomposition of the ammonium form using a high temperature oxidative treatment.
The activity and selectivity of hydroisomerization catalysts depend on a variety of factors, such as the mode of catalyst preparation, the presence or absence of promotors, quality of raw materials, feedstock quality, process conditions, and the like. Suitable catalysts can be conventionally prepared by combining commercially available crystalline zeolites, such as, a hydrogen form mordenite, with a suitable matrix material followed by the addition of a Group VIII metal, and thereafter activating by conventional means.
The process of isomerizing hydrocarbons containing high levels of sulfur can now be effectively accomplished utilizing a catalyst of novel composition. As exemplified within the instant application, this catalyst demonstrates superior isomerization performance compared to conventional isomerization catalysts when processing feeds containing sulfur.